**2. Description of numerical model**

### **2.1 Physical model**

The research object of this chapter is made of polyethylene (PE) pipe, which is applied in Chongqing, China. The construction process is strictly in accordance with the relevant requirements of *Technical Specifications for Ground Source Heat Pump System Engineering*. And the specific parameters are shown in **Table 1**.


**Table 1.**

*Main structural parameters of HGHE.*

Using the Design Modeler module in ANSYS\_FLUENT, which is a professional finite element analysis tool, this work established the geometric model of HGHE. According to the real size, the surrounding soil was set as a cylinder in the physical model. And the geometric model is shown in **Figure 3a**.

Then, the meshing module of ANSYS\_FLUENT software is used to divide the grid of the built geometric model of HGHE. The grid division follows the following principles: If the temperature field or velocity field in a certain position and direction has significant fluctuations, the area should be divided into dense grids. On the contrary, if the fluctuation of temperature field or velocity field is not obvious, the meshing of this area can be loosened appropriately. Because the scale of fluid flow and heat transfer in the spiral buried pipe are small and fluctuate greatly during heat transfer, the grid of the spiral buried pipe and its nearby soil area are encrypted, as shown in **Figure 3b**. The specific grid division is as follows:


#### **Figure 3.**

*Mesh division of physical model of HGHE. (a) Physical model of HGHE. (b) Mesh division of HGHE.*

3.Mesh with higher quality can accelerate the convergence speed, reduce the error in solving, avoid the divergence of numerical simulation results, and ensure the accuracy of results. ANSYS\_FLUENT software is required to ensure that the minimum orthogonal quality is greater than 0.1 or the maximum inclination is less than 0.95. Most of the grids in the model established in this chapter have good quality, and a few grids with poor quality are in the acceptable range of ANSYS\_FLUENT. Therefore, the grids used for numerical calculation can meet the requirements of ANSYS\_FLUENT.
